Fused silica is highly relevant to such applications as windows and mirrors used in outer space, and increasingly, it is becoming relevant to optical elements for deep ultraviolet photolithography. However, it is generally known that prolonged exposure of fused silica to intense deep ultraviolet radiation of the type utilized in photolithography leads to optical damage which is generally manifested in the form of changes in the optical and physical properties of the glass.
Laser-induced optical absorption is a commonly observed problem with fused silica. In addition to induced absorption and perhaps more importantly, there is also observed in fused silica glass a physical densification or compaction of the exposed regions of the glass when exposed to high energy irradiation. Lens elements of a stepper (for photolithographic applications) which utilize deep ultraviolet wavelengths for high resolution microcircuit fabrication may become altered due to optical modification as a consequence of prolonged exposure. Even though small changes in the optical phase front produced by the effect of exposure over the life of the lens barrel are expected, at present the maximum acceptable change is not known. What is known however, is that there is a relationship between alterations in fused silica and the ultimate effect of such changes on the wavefront. The present work is directed towards a better understanding and characterization of these relationships. Compaction or densification is most readily observed by interferometry where the alteration of the optical phase front is measured through the damaged region. Usually reported as optical path length difference, OPD, densification is measured as the product of the refractive index and the path length, in parts per million.
The question of what factors contribute to the propensity of various silica materials to optical damage when irradiated with high energy laser is not settled and several possible answers have been advanced in the literature.
In the past, it has been suggested that high OH content is desirable for low induced absorption. However, high OH fused silica is not always practical because certain applications require little or no OH, for example, waveguide applications. As a result, recently it has been suggested in co-assigned U.S. Pat. No. 5,616,159 that induced optical absorption can be significantly controlled in fused silica glass regardless of the OH content by subjecting the glass to a molecular hydrogen treatment. In that connection, it has also been disclosed in co-pending, co-assigned U.S. patent application Ser. No. 08/697,094, a low OH (less than 50 ppm) fused silica glass which is highly resistant to optical damage up to 10.sup.7 pulses (350 mJ/cm.sup.2) at a laser wavelength of 248 nm.
In co-pending, co-assigned U.S. application Ser. no. 08/762,513, it was suggested that high purity fused silica glass having high resistance to laser-caused optical damage can also be produced by diffusing out of the glass, molecular oxygen
More recently, in co-assigned, co-pending PCT patent application Ser. No. PCT/US97/11697, deposited Jul. 1, 1997, titled "Fused Silica Having High Resistance to Optical Damage," it was suggested that radiation-caused optical damage can be minimized or eliminated by precompacting fused silica by such processes as hot isostatic pressing and by high energy pre-exposure in order to thereby desensitize the glass to subsequent high energy irradiation during actual use.
To the best of our knowledge, until now there has been little or no discussion in the literature about the cause of the induced compaction (densification), or of how this propensity to compact can be predicted in the first instance. Accordingly, it is the object of the present invention to provide a model for predicting compaction in fused silica, as well as a method for identifying glass which will be resistant to compaction.